288 THE BIOLOGY OF THE FROG chap. 



of which connections become established between different 

 parts of the cord. In the broad ventral cornua there are 

 several ganglion cells of unusual size from which processes 

 arise which form the axis cylinders of the fibers of the ven- 

 tral roots of the spinal nerves ; other processes from these 

 cells cross to the opposite side of the cord in the ventral 

 white commissure, and still other processes branch irregu- 

 larly in both the gray and white matter of the same side. 

 Scattered about through most of the gray substance are the 

 commissural cells which give off axis cyHnder processes 

 which cross to the opposite side of the cord in the ventral 

 gray commissure and then give off branches which run in 

 the white matter both anteriorly and posteriorly ; protoplas- 

 mic processes are also given ofT which connect with similar 

 processes from other cells in the gray matter in the same 

 side. Other cells give off axis cylinder processes, which run 

 in both directions in the white matter of the same side of 

 the cord. Still other cells occur whose axis cylinder pro- 

 cesses divide, the one branch going into the white matter of 

 the same side of the cord, the other crossing through the 

 ventral gray commissure to the white matter of the opposite 

 side. Finally there are numerous cells whose processes do 

 not enter the white matter, but branch and connect with 

 cells in the gray matter of the same or the opposite side. 



A cross section through a region where the spinal nerves 

 are given off shows the fibers of the dorsal root passing 

 through the dorso-lateral portion of the white matter to enter 

 the gray substance in a narrow bundle. Most of the fibers 

 of the dorsal roots are processes of cells lying in the spinal 

 ganglion. Each fiber as it enters the cord gives off branches 

 which run in opposite directions. Connections are made 

 with processes of the large cells which supply the ventral or 

 motor roots of the nerves as well as with the cells of the gray 



